Two-way wireless communication systems that support inbound calling from the public switched telephone network are well known in the art. Examples of such systems include cellular telephone systems and trunked radio systems. Also becoming well known are city-wide telepoint systems based on second generation cordless telephony (CT2) technology. This latter technology provides service within small, isolated "islands of coverage" each defined by a limited radio coverage range provided by one or more fixed communication units located therein.
Unlike cellular telephone systems and trunked radio systems, CT2 city-wide telepoint systems typically have hundreds or even thousands of fixed communication units, each individually interconnected with the public switched telephone network. With so many fixed communication units the routing of calls inbound to portable communication units being carried about the islands of coverage becomes a problem, because the system must locate the fixed communication unit nearest a called portable communication unit at the time of each inbound call to connect the call.
It is possible to conceive of several methods that might be used for locating the fixed communication unit nearest a called portable communication unit at the time of an inbound call, but it is difficult to conceive of a method that is not accompanied by substantial drawbacks. For example, a user could execute a manual registration key sequence upon arrival at a destination to indicate that the system should forward inbound calls to the destination. Manual registration, however, is considered by many to be a bothersome process. Furthermore, manual registration is impractical for users while on the move.
Critics of manual registration perhaps would suggest that the portable communication unit could monitor some form of a station identification signal transmitted by the strongest fixed communication unit and (re)register automatically if the station identification changes to that of a different station. Unfortunately, automatic registration would shorten battery life in the portable unit because of the required frequent registration transmissions from the portable communication unit. Automatic registration also can place a high traffic load on a central portable communication unit location registry as users move about the hundreds or thousands of fixed communication units typically found in a city-wide CT2 system.
Critics of both manual and automatic registration would perhaps propose sending call set-up polls from every fixed communication unit in a system, but doing so in all but the smallest of conventional systems would badly overload the fixed communication units with polling traffic, thus usurping costly transceiver capacity that would be better used for user communications.
One method that has been proposed is to use a fast city-wide radio paging system for sending a page to a portable communication unit that includes an integral radio pager. After receiving a page the portable communication unit links with a nearby fixed communication unit and reports its presence. This method, while a workable solution where sufficient radio paging capacity is available, is not practical everywhere. Radio paging also is a relatively expensive solution.
Thus, what is needed is a method of locating a portable communication unit to complete an inbound call for the portable communication unit without the disadvantages that accompany conventional methods. A method is needed that does not require either bothersome manual registration by the user or power consuming frequent automatic registration by the portable communication unit. In addition, a method is needed that does not place a high traffic load on a central portable communication unit location registry. Furthermore, a method is needed that does not require a fast city-wide paging system and that does not overload the fixed communication units with polling traffic.